1. Field of the Invention
The present invention relates to an image forming apparatus capable of detecting a malfunction of a mechanism that determines a malfunction of an element or a device used for image forming processes and also detecting a malfunction of a charging unit before these malfunctions may lead to an abnormal image.
2. Description of the Related Art
Image forming apparatuses such as copiers, printers, or facsimiles and printing machines can obtain a printout in such a manner that a developing unit visualizes an electrostatic latent image formed on a photosensitive element that is a latent-image carrier and a visualized image is transferred to a sheet or the like.
Japanese Patent Application Laid-open No. S63-289563 discloses a configuration of performing self-diagnosis of devices and elements used in an image forming process in an image forming apparatus.
In the disclosed configuration, by comparing each current value at devices other than a paper feeder when load is applied thereto based on actual image forming conditions with a reference value and when the detected current value falls within an allowable range, it is determined that there is no malfunction. In this method, however, the self-diagnosis is performed on the image forming apparatus only upon start of operation thereof, and if the result of the self-diagnosis at that time is satisfactory, then it is just determined that there is no malfunction. Thus, it is understood that the method is not configured to predict occurrence of a malfunction of the image forming apparatus.
Therefore, even if no malfunction occurs under operational conditions at the current stage, it is impossible to predict and determine the occurrence of a malfunction when the devices may be degraded due to continuation of the state and affected by environmental fluctuations. Consequently, if a malfunction occurs, then it is urgently required to response to the occurrence of the malfunction, and the imaging operation may thereby be interrupted. Especially, the device used in the charging process that is inevitably executed to form an image cannot set a predetermined potential on the photosensitive element caused by abnormal charging due to degradation of the device with the passage of time. In this case, if the background potential of the photosensitive element is different from a predetermined value, then the density of an image may be decreased.
The charging process in an electrophotographic device is a process of uniformly charging a specific dielectric element that is a photosensitive element or a latent-image carrier. A method of charging the photosensitive element is roughly classified into two methods such as a non-contact charging method and a contact charging method. A now commonly used charging method is a corona charging method typically used in a corotron charger and a scorotron charger which are included in the non-contact charging method.
However, the chargers using the corona charging method produce ozone. Therefore, due to ozone regulation based on recent environmental problems, the mainstream of the corona charging method used in low speed and intermediate speed electrophotographic devices is shifting to the contact charging such as a roller charger and a brush charger which produce a small amount of ozone. To achieve more uniform charging, the scorotron charger is provided with a grid near a wire through which an electrical current passes, and the roller charger is configured to superimpose an alternating current (AC) on a direct current (DC).
Japanese Patent Application Laid-open No. 2004-345091 describes a technology for dividing an input image and an output image into n blocks to be subjected to color shift correction and color correction of images, performing pattern matching for each divided block, and comparing patterns. However, in this technology, only current quality of an image forming apparatus can be obtained, and thus it is impossible to predict that an image will become abnormal such as color shift in ordinary use.
Japanese Patent Application Laid-open No. 2005-176045 describes a technology for reading an abnormal-image detection chart by an image reader to identify which of a printer side and a reading side has a factor of an abnormal image such as inclination. However, in this technology also, only current quality of an image forming apparatus can be obtained, and thus it is impossible to predict that the image will become abnormal such as color shift in ordinary use.
However, the following problems remain in the charging process. More specifically, in the non-contact charging and the contact charging, a charging failure due to electrical discharge and dirt may occur with the passage of time in a portion such as a wire or a roller used to directly charge a photosensitive element. The charging failure causes a line to appear also in a sub-scanning direction of an image usually as a color streak. The portion where the charging failure has once occurred is difficult to recover, and the portion becomes a factor of short life of a unit to which a charging unit belongs. Occurrence of an abnormal image due to the charging failure does not allow to obtaining a normal image.
Furthermore, to prevent an abnormal image due to the charging failure, the life of the unit has to be decided so as to have a certain allowance. This does not make maximum use of the life, which leads to an increase in costs.